UK 1,513,966 describes electromagnetic switching devices, FIGS. 10 and 11 of which show such a device in which an armature 3 can rock between two points under the influence of electric current produced changes in the flux linking the armature and four poles 6, 6′ 7 and 7′. In FIG. 11 springs are shown permanently linking one end of the armature two of the poles, to centre the armature between the poles. Examples of the centring springs are shown in FIGS. 12 and 13.
In both devices shown in FIGS. 10 and 11 of UK 1,513,966 a layer of non-magnetic material from which the coil bobbin is constructed, exists between the ends of the magnets and the magnetic material forming the poles 6, 6′, 7 and 7′ respectively. This significantly weakens the flux density available to attract and hold the armature in contact with poles 6 and 7′ (or 6′ and 7). In fact, unless current is flowing in the solenoid coil of the FIG. 11 device, the armature appears to be held by the centring springs midway between the poles, out of contact therewith. That device is therefore not a truly bistable device, but can adopt an intermediate third state, in which the armature makes no contact with any of the poles. In fact since the springs shown centre the armature in the absence of current in the coil, the forces exerted by the springs on the armature are greater than the force of attraction due to the permanent magnet, and it is only when the magnetic force acting on the armature is increased by the flux due to the current flowing in the coil, that the spring forces are overcome and the armature will move into contact with one or the other set of poles 6 and 7′ or 6′ and 7.
An electromagnetic actuator is described in U.S. Pat. No. 4,621,660 which employs permanent magnets, a pivoting armature and a winding through which an electric current is passed to shift the armature from one position to another. The armature co-operates with ports and passages in a housing containing the actuator so as to cover and uncover openings therein to control the flow of fluid from one passage to another. However no attempt is made to store energy during armature movement, subsequently to accelerate the armature in transit between its two positions. The actuator does not therefore have a fast response time. In addition the armature will impact against the newpoles as it transfers from one pair of poles to the other, and this can introduce significant wear and shorten the life of the device.